This invention relates generally to telecommunications devices such as radiotelephones and, in particular, to radiotelephones or mobile stations that operate in accordance with a digital data transmission format and protocol.
In one modern digital telecommunications system based on an air interface standard known as IS-136, a received data field in a digital data channel (DDCH) includes a transferred ordinary data block (DATA) or a control and supervisory message, known specifically as the fast associated control channel (FACCH). The IS-136 standard is one based on time division/multiple access (TDMA) and uses a slotted frame structure for both a forward channel (base station to mobile station) and a reverse channel (mobile station to base station).
Referring briefly to FIG. 3, it can be seen that a traffic channel can contain user information such as DATA, the FACCH, and a slow associated control channel (SACCH). As defined in IS-136, the traffic channel can contain at any given time only the user information or the FACCH, not both simultaneously. The DATA and FACCH use different encoding/decoding methods (and cyclic redundancy checks (CRCs)). The DATA may be data bits, such as facsimile data or computer network data, or may represent encoded speech data. The FACCH is defined as a channel used for signalling message exchange between the base station and the mobile station. Reference can be had to IS-136.2, Rev. A., Feb. 12, 1996, Section 2.7.3.1.1 (Fast Associated Control Channel (FACCH)), and to subsections 2.7.3.1.1.1 to 2.7.3.1.1.5 contained therein, for a description of the FACCH that is of most interest to this invention.
In general, the FACCH data is error protected by means of a rate 1/4 convolutional code, and 49 data bits in a FACCH word is appended with a 16-bit CRC to detect the presence of channel errors. Another intended purpose for the 16-bit CRC was originally stated to provide a mechanism to distinguish FACCH data from speech data. However, this latter purpose has been found to not be adequately fulfilled.
A general problem has arisen as to how to most accurately detect the data (and data type) in the received signal decoding process. Typically the data is decoded using one or more decoding methods, and then if a data integrity test passes (typically one using a CRC), the decoded bits become available for further processing. If the CRC check fails then another method may be tried for the decoding process, and another test of the CRC is made.
More particularly, it has been determined that an occasional received DATA block, with certain bit combinations, may erroneously pass the FACCH decoding process, and the FACCH CRC check as well. This failure mode is due to the rather limited length of the CRC field. In such a case the DATA may be lost, since it was erroneously detected to be a FACCH message from the base station.
In greater detail, a received block is typically decoded first as the FACCH. It has been found, however, that the FACCH CRC check will sometimes pass with some bit combinations that are found in ordinary data blocks (e.g., encoded speech data). In this case the data block is detected (erroneously) to be a FACCH message, and the ordinary data can be lost completely. The presumed FACCH message is then further processed and subsequently detected to be invalid (erroneous type). Also very troublesome is the case where the assumed FACCH message (actually data) corresponds to one of the plurality of valid FACCH messages, thereby introducing a possibility that the mobile station may operate in an erroneous manner. In any event, the intended ordinary DATA block has been lost.
There may exist an ability to retransmit the DATA (e.g. if no acknowledgement of receipt by the mobile station is detected at the base station), especially in some circumstances where the DATA is regarded as vital. However, this could result in the generation of an endless loop if the DATA is again erroneously detected as a FACCH message in the mobile station. The final result could be dropping a DATA call.
The same failure mechanism can be found on the base station side as well, and a DATA call can be dropped when the DATA is being transferred to the base station from the mobile station.
Similar failure mechanisms have been observed with VSELP-coded speech, where a received FACCH message was erroneously detected as speech due to the CRC check being passed.
To summarize, and by example, in an IS-136 specification-based mobile station the FACCH data is well protected against channel induced errors (1/4 rate convolution coding), and the decoding process can correct several erroneous bits in the data field. Typically, but not necessarily, the mobile station first attempts to decode the received signal as a FACCH message. However, if the decoded data block is not a FACCH message (i.e., it is ordinary DATA) the FACCH decoding process attempts to correct several (typically 30-65) bits in the data field. In most cases the CRC check will fail. However, some random bit sequences in decoded (ordinary) DATA may also produce a CRC check that passes. The decoding process then erroneously assumes that the received bits represent a FACCH message that has a high bit error rate (poor RF signal), and the DATA is lost. The presumed FACCH message will then typically be subsequently determined to be invalid, although in some cases the received bit sequence may decode to a valid FACCH message, resulting in a possibility of erroneous mobile station operation.
It is thus a first object and advantage of this invention to provide an improved method for receiving information from a traffic channel in either or both of a mobile station and a base station of a radio telecommunications system.
It is a further object and advantage of this invention to provide a method for distinguishing received control messages from received data, such as computer data or encoded speech data.
It is another object and advantage of this invention to provide a method for distinguishing received FACCH messages from received user DATA, such as computer data or encoded speech data.
The foregoing and other problems are overcome and the objects and advantages are realized by methods and apparatus in accordance with embodiments of this invention.
In accordance with the teachings of this invention a decoding process is carried out with all possible decoding methods. If the CRC check passes with more than one decoding method, then a further study of the decoding process is made to determine which one resulted in a correct CRC check.
In accordance with an embodiment of this invention the receiver circuitry typically first decodes a received signal as a FACCH message and obtains a first BER, and then also attempts to decode the same input signal (received bits) as ordinary or user DATA (DATA in a digital data channel and speech in a digital speech channel). In the latter case the decoding process and the CRC check will also pass, because the received data was actually an ordinary DATA block. The decoding process then corrects, typically, zero or only a few erroneous bits (induced by a poor radio channel) and it finds that a second BER is significantly lower than the first BER that was obtained when the received signal was decoded assuming a FACCH message. The data type (FACCH or ordinary (user) DATA) is then selected based on the first BER obtained when the received signal was decoded as a FACCH message and the second BER obtained when the received signal was decoded as ordinary DATA. That is, it is assumed that the lowest BER indicates the true data type of the received signal.
More particularly, in an embodiment of this invention a method is disclosed for operating a mobile station of a type that receives a digital traffic channel, such as a time division, multiple access (TDMA), IS-136 compliant mobile station. The method includes steps of (a) receiving a signal from a forward traffic channel and decoding the received signal using a first predetermined decoding technique to generate a first bit error rate (BER); (b) decoding the received signal using a second predetermined decoding technique to generate a second BER; (c) comparing the first BER to the second BER; and (d) declaring the received signal to be one of a FACCH message or user data depending on a result of the comparison. The step of declaring may include a preliminary step, for a case where the step of comparing indicates that the received signal is a FACCH message, of first verifying that the received signal contains a valid FACCH message type.
In other embodiments of this invention, useful in either one or both of a mobile station and a base station, a combination of signal quality indications, such as BER and CRC validity indicators, are used to detect a type of data that is received, and to declare that the received data is one of a control and supervision related message (such as the FACCH), or is user DATA or encoded speech information.
In general, the teachings of this invention apply to both DATA calls and to voice calls.